1. Field of the Invention:
This invention relates to pre-foamed particles of an uncrosslinked, linear low-density polyethylene.
2. Description of the Related Art:
So-called articles expansion molded from foamable beads (expansion-molded articles), which are obtained by filling pre foamed particles in individual molds and then heating them, thereby expanding the pre-foamed particles, have been utilized in wide-spread applications such as bumping (shock absorbing) materials, packaging materials, heat insulating materials and building materials owing to their excellent bumping properties, heat insulating properties, etc. Their demand has hence increased largely in recent years.
Articles molded from foamed polystyrene particles have been known as such expansion-molded articles. The expansion molded articles of the polystyrene beads are however accompanied by a fatal drawback that they are brittle and moreover, by another drawback that they are poor in chemical resistance. It has hence been desired to improve such drawbacks from early on. As those for solving such drawbacks, there has been proposed articles molded from foamed polyethylene particles. However, since polyethylene resins remarkably decrease in viscosity near theIr melting points, crosslinked polyethylene resins have usually been employed. If pre-foamed particles of the crosslinked polyethylene resin are used, it is however difficult to obtain any low-density (high-expanded) molded articles by expansion molding. If trying by force to obtain low-density molded articles, molded articles, which undergo significant shrinking and have high water absorbency and poor physical properties, are obtained only. Therefore, it is quite impossible to obtain low-density molded articles of the polyethylene resins, which can be put to practical use. In addition, although a high-pressure-processed low-density polyethylene is principally used as a base material for the crosslinked polyethylene owing to its good crosslinkability, the high-pressure-processed low-density polyethylene has poor heat resistance and insufficient stiffness. Thus, its expansion molding must of necessity be conducted at a relatively low expansion ratio.
As a method for solving such problems, it has been proposed in Japanese patent publication No. 10,047/1985 to mold pre-foamed particles of an uncrosslinked, linear low-density polyethylene. The pre-foamed particles of the uncrosslinked polyethylene are however accompanied by such problems that secondary foaming cannot be achieved sufficiently and excellent molded articles cannot hence be obtained unless expanding ability is imparted to the pre-foamed particles because of a fact that their heating temperature range upon molding is limited and it is hence impossible to heat them sufficiently, and the crystalline structure of the uncrosslinked, linear low-density polyethylene. When the pre-foamed particles of the uncrosslinked, linear low-density polyethylene is molded, it is therefore used to feed an additional supply of a gaseous foaming agent or inorganic gas such as air to the pre-foamed particles prior to the molding, thereby building up an internal pressure therein. The feeding of an additional supply of the gaseous foaming agent or inorganic gas to the pre-foamed particles is however accompanied by a problem that it results in considerable loss in both equipment and expense, and the production cost of molded articles comes expensive. Furthermore, pre-foamed particles of polyolefin resins generally tend to dissipate gas in cells. It is hence difficult to hold foaming ability for a long period of time even if the pre-foamed particles are imparted with foaming ability by additional supply of inorganic gas or the like to build up an internal pressure therein. In order to obtain excellent molded articles by these conventional techniques, it is hence necessary to consume the pre-foamed particles within a short period of time subsequent to the build-up of internal pressure. They are by no means considered to have been completed to such an extent that many molding companies can produce molded articles easily from pre-foamed particles furnished by pre-foamed particles makers
On the other hand, it is disclosed in Japanese Pat. Publication No. 7816/1980 to obtain a molded article by molding pre-foamed particles of a crosslinked polyethylene resin without need for the pretreatment to build up an internal pressure therein by additional supply of inorganic gas or the like, cooling the contents to a temperature ranging from the softening temperature of the resin (exclusive) to room temperature, heating the contents to a temperature ranging from (the softening temperature of the resin -40.degree. C.) to the softening temperature (exclusive), and then cooling the contents gradually. The pre-foamed particles of the crosslinked polyethylene resin is hoWever accompanied by a problem that it is difficult to obtain low-density molded articles as described above even if they can be molded without need for any pretreatment to build up an internal pressure. All the techniques described above were satisfactory insufficiently.